wcc: -T test-mode collects @test fns + synthesizes entry, both stages (#15)

@test was parsed then dropped (no consumer); `ww test` needed a hand-written
main listing each test by hand, so adding a @test and forgetting the call
silently skipped it. -T makes the checker collect @test N_FNDECLs in source
order, loud-reject a user main, and append a synthetic
`export fn main() i32 { t0(); ...; return 0; }` at the install->body-check seam;
the existing cgfn emits it (cgen untouched) -> byte-identical by construction.
Mirrors harec's checker-side is_test placement.

Plan-9-lean reduction (user-sanctioned, reinstatable post-CSP): sequential,
abort/nonzero=fail; no setjmp isolation, no fnmatch filter, no file:line.

910/997 rewired from a regex scanner to driving `w6c -T` directly (thin trusted
drivers; the @test content stays ww), with a cross-stage byte-id assert on the
-T output. attest_userman/attest_badsig pin the user-main and bad-signature
rejects.

cmd/w6c/main.c

@@ -33,10 +33,13 @@ main(int argc, char **argv)
 {
 	const char *src = NULL;
 	const char *out = NULL;
+	int testmode = 0;
 	for (int i = 1; i < argc; i++) {
 		const char *a = argv[i];
 		if (strcmp(a, "-o") == 0 && i + 1 < argc) {
 			out = argv[++i];
+		} else if (strcmp(a, "-T") == 0) {
+			testmode = 1;
 		} else if (a[0] == '-') {
 			fprintf(stderr, "w6c: unknown flag %s\n", a);
 			return 2;
@@ -48,7 +51,7 @@ main(int argc, char **argv)
 		}
 	}
 	if (src == NULL) {
-		fputs("usage: w6c [-o out.s] file.ww\n", stderr);
+		fputs("usage: w6c [-T] [-o out.s] file.ww\n", stderr);
 		return 2;
 	}
 
@@ -71,6 +74,7 @@ main(int argc, char **argv)
 	if (l.errs || p.errs) return 1;
 
 	check_init(&c, a);
+	c.is_test = testmode;
 	check_file(&c, file);
 	if (c.errs) return 1;